The aperture required for a CRT display in the wall of a shielded enclosure containing electromagnetic generating components creates a potential shielding deficiency. Electromagnetic energy generated by the components may be transmitted through the aperture with relatively minor attenuation. The conventional approach used for attenuating emissions through the aperture is an optically transparent conducting screen in the form of either a wire mesh or a thin homogenous layer, for example indium tin oxide or metallic gold. The conducting screen covers the external face of the CRT and is bonded along the entire periphery of the aperture, thus providing a shielding enclosure completely enclosing the emission generating components.
The use of external screens causes a degradation in pixel pitch, resolution and the brightness of the CRT display. In addition, the formation of "noise" patterns occurs as a consequence of interaction of the geometric periodicity of the wire mesh and the dot matrix CRT phosphor geometry. These problems are pronounced in high resolution color monitors. The use of homogeneous conductive films significantly reduces optical transparency at thicknesses required to provide adequate shielding. The utilization of either a wire mesh or a thin homogeneous layer requires electrical bonding to the conductive monitor enclosure which is expensive and often increases the weight and physical size of the CRT display. In addition, the appearance and operational ergonomics of the monitor are degraded. As optical resolution and video bandwidths increase, the limitation of external shielding of a color CRT display becomes more evident.
One approach utilized to inhibit electromagnetic emissions through the aperture and face of the CRT display without the use of external conducting screens is disclosed U.S. Pat. No. 4,767,969, entitled RF EMISSION SHIELD FOR CRT DISPLAYS, issued Aug. 30, 1988 to Green and assigned to Honeywell, Inc. Disclosed therein is a shield placed at the neck of a CRT envelope which inhibits the RF emissions from being transmitted through the aperture and face of the CRT display. The shield comprises longitudinal conductors and circular conductors for suppressing emissions from the CRT.
Although a single shield method, such as that disclosed in U.S. Pat. No. 4,767,969, reduces the electromagnetic emissions through the CRT display face, further suppression techniques to meet compliance levels without the use of external screens are needed.